Remove support for nested mosaics

[kylebarron]

In places like

cogeo-mosaic/cogeo_mosaic/backends/utils.py

Lines 59 to 69 in d1b6794

	# check if we have a mosaic in the url (.json/.gz)
	return list(
	itertools.chain.from_iterable(
	[
	get_assets_from_json(tiles, quadkey_zoom, x, y, z)
	if os.path.splitext(asset)[1] in [".json", ".gz"]
	else [asset]
	for asset in assets
	]
	)
	)

and

cogeo-mosaic/cogeo_mosaic/backends/dynamodb.py

Lines 223 to 227 in d1b6794

	assets = list(
	itertools.chain.from_iterable(
	[self._fetch_dynamodb(qk).get("assets", []) for qk in quadkeys]
	)
	)

there's inherited old code that supports nested mosaics, where I think an asset of a mosaic can reference another mosaic?

Since this isn't documented anywhere, I think we should remove it, unless you're still using this internally? I think it would make the code a lot clearer